1. Field of the Invention
Exemplary aspects of the present invention relate to a semiconductor device and a semiconductor laser driving device, and more particularly, to a semiconductor device and a semiconductor laser driving device for driving a semiconductor laser used for an optical writing unit of a laser printer, optical data communication, an optical disk, and the like.
2. Description of the Related Art
A conventional method of controlling an amount of light emitted by a semiconductor laser used for image writing using an image forming apparatus is the so-called APC (automatic power control) method. In the APC method, a photosensor is provided near a semiconductor laser to periodically detect a laser beam emitted by the semiconductor laser, and a signal detected by the photosensor is fed back to a semiconductor laser driving circuit as a feedback signal to maintain an amount of light emitted by the semiconductor laser at a predetermined level.
FIGS. 1 and 2 are graphs illustrating a relation between a drive current and an amount of light emitted by a semiconductor laser. Generally, a drive current of the semiconductor laser is obtained by combining a bias current Ibi and a switching current Isw, because the relation between the amount of light and the drive current of the semiconductor laser is nonlinear, as illustrated in FIGS. 1 and 2.
That is, the semiconductor laser emits only a small amount of light when the drive current does not satisfy a threshold level current Ith, but after exceeding the threshold current Ith, the amount of light emitted by the semiconductor laser increases as the more the drive current exceeds the threshold current Ith (light-emitting current Iη).
When the drive current of the semiconductor laser increases from zero to a level required to emit a predetermined amount of light, it takes a long time for the semiconductor laser to start to emit light. Alternatively, the semiconductor laser may be continuously supplied with a bias current Ibi close to the threshold current Ith, and an amount of a switching current Isw that corresponds to a difference between the drive current and the bias current Ibi is switched on and off to provide high-speed, precision control of laser light emission.
More specifically, FIG. 1 illustrates the relation between a drive current and an amount of light emitted by a semiconductor laser greatly fluctuating in a threshold current Ith. For example, as the semiconductor laser heats up the threshold current Ith changes and increases to a threshold current IthH, as indicated by line B. Therefore, in order to emit the same amount of light, the semiconductor laser needs a larger threshold current IbiH.
FIG. 2 illustrates the relation between a drive current and an amount of light emitted by a semiconductor laser greatly fluctuating in light emitting efficiency. Note that the light emitting efficiency Is a relation between a light-emitting current In and an amount of light. In this example, the light-emitting efficiency decreases as the semiconductor laser heats up, as indicated by line C. Therefore, the semiconductor laser needs a larger switching current IswH in order to emit the same amount of light.
Therefore, some related-art semiconductor laser driving devices using the APC method control the bias current Ibi based on a signal detected by a photosensor. However, such semiconductor laser driving devices are not appropriate for a semiconductor laser with large fluctuations in light emitting efficiency like those illustrated in FIG. 2.
Other related-art semiconductor laser driving devices control both the bias current Ibi and the switching current Isw based on a signal detected by a photosensor. However, such semiconductor laser driving devices have a complicated circuit configuration and thus are expensive.
It is also to be noted that, when conventional light amount control circuits are integrated into a single IC (integrated circuit) chip, it is difficult to easily change circuit configuration according to variations in size, temperature characteristic, or the like of each semiconductor laser.
Accordingly, there is a need for a technology to provide a semiconductor device and a semiconductor laser driving device capable of flexibly accommodating different circuit configurations depending on specific characteristics of each semiconductor laser.